Generally, composite structures are made up of multiple elements. And often, these elements intersect and are interconnected at joints. In this regard, the integrity of these joints is typically critical to the performance of the structure. One very common joint is the T-joint, where one plane of the composite ends at the surface of another. Another, less common joint, is the X-joint, in which two planes intersect one another. For design reasons, the intersections of these elements have some type of external radius (i.e., the intersection does not have sharp corners), and contain an internal filler material, sometimes referred to as a “noodle.” The quality of the noodle, its interface with the elements of the composite structure, and the consolidation of the elements, are all critical to proper joint functioning. For example, in the aircraft industry, the quality of the intersection of the webs and flanges in composite spars, or webs and skins in co-cured structures are critical to their performance. Flaws, such as cracks, voids, delaminations, or porosity can form in the joint region and adversely affect the composite structure.
To help ensure the integrity of joints in composite structures, the joints are generally inspected for flaws. Such joints are typically very difficult to inspect, however, particularly using traditional nondestructive inspection (NDI) methods. The techniques that can “see” joint defects require lab-intensive inspections or the use of multiple axis robotic scanners. For example, one NDI method of inspecting radiused joint regions includes using a hand-operated ultrasonic testing (UT) transducer in pulse-echo mode with a radiused shoe mounted on its end. The operator holds the shoe against the inner radius of the joint, sliding it along the length, and rocking it back and forth over a near 90° angle. The operator looks for flaw indications that will affect the ultrasound back to the transducer, which will be picked up and indicated by changes in the amplitude/time trace on an oscilloscope. The operator must determine “on the fly” whether or not the UT reflection amplitude is high enough and, at the same time, whether the extent of the flaw is great enough to disqualify the composite structure. In this regard, the operator generally utilizes a radius flaw standard and a pre-determined NDI criteria for flaw amplitude and length.
While NDI methods including hand-operated UT transducers are adequate for detecting flaws in composite structures, such methods suffer from numerous drawbacks. First, such methods are generally costly and time consuming as the operator is required to operate the hand-operated UT transducer during the entire process. Second, such methods are operator dependent and, as such, are subject to potential operator errors. In this regard, the operator must continuously monitor an oscilloscope for signal changes, while moving the transducer in the radial and axial directions along the joint region. Further, flaw indications are often subtle and, therefore, require tracking at multiple angles with complete coverage often difficult to ensure.
Third, such hand-operated inspection methods do not provide reviewable image data. In this regard, no data is saved for subsequent analysis or review if questions arise subsequent to the inspection. Fourth, such methods do not produce images that show the size or length of any flaw indications that are discovered. The operator simply marks the measured length of an indication on the part itself. Finally, due to the extremely high attenuation of ultrasound by air, a couplant, such as water or a semi-liquid gel, is generally required between the transducers and the composite structure for the UT transducers to work properly. The inclusion of the couplant, however, limits inspections to structures that will not be contaminated by the couplant.
In light of the drawbacks to hand-operated inspection methods, a number of automated (i.e., machine-driven) methods have been developed. One such automated, method, the multiple-transducer automated UT scanning system, makes use of several transducers mounted in a variety of orientations. In this regard, such systems are typically set up for a particular configuration of a particular composite structure. multiple-transducer automated UT scanning systems generally work well for straight, lengthy parts, but they are expensive to manufacture and are relatively inflexible. Such systems also require water squirters to supply couplant between the transducers and the composite structures.
Another automated system makes use of a rotating mirror scanning head mounted to an x-y-z robot to inspect radii. This system eliminates most of the drawbacks of the aforementioned methods, but it too is expensive to implement. Further, such a system is generally difficult to operate, and is sensitive to radius orientation (relative to scanning head) and surface roughness, which affects the ability of the system to couple to the composite structure to receive reliable data.